1. Field of the Invention
The invention in general relates to vehicle monitoring or control systems and particularly to an optical scanning system therefor.
2. Description of the Prior Art
A wide variety of traffic control or monitoring systems exists wherein the location of a vehicle is continuously reported to a central location during the course of travel of the vehicle. Such systems satisfy the needs of a municipality with regards to bus transit, police operations, fire departments, taxis, trucking, and municipal services.
Existing or proposed vehicle location systems include radio navigation systems, dead reckoning systems, and signpost systems. Signposts are classified as active when they are powered, and continuously emit a recognition signal irrespective of the absence or presence of a vehicle which can read the signal. Signposts are classified as passive when they emit a recognition signal only when triggered by an initiating signal emitted by a nearby vehicle. Due to the initally lower installation and maintenance cost, the passive signpost system appears to be an attractive approach. In one proposed passive system such as described in Traffic Engineering & Control, December 1970 beginning at page 410, an optical scanner is mounted on the vehicle and a beam of light projected out one side of the vehicle vertically scans a uniquely coded signpost generally consisting of a series of reflecting and non-reflecting bars arranged in a vertical sequence. The return light beam is thus modulated in accordance with the coded signpost and this information is detected and transmitted to a central location.
In another system such as illustrated in U.S. Pat. No. 3,940,630, scanning of a coded signpost is accomplished by movement of the vehicle. The arrangement produces a fan-shaped beam which reads a horizontally oriented coded signpost as the vehicle moves past it.
Since a wide variety of vehicles normally exist on a crowded city street, the optical beam, whether it is a horizontally moving fan-shaped beam or a vertically scanning narrow beam may be unable to read a particular signpost if a vehicle of greater height is blocking the optical path. It is conceivable, particularly on crowded streets, that the blockage may occur for several successive signposts. With this situation, or if the vehicle fails to read a signpost immediately after a turn, an objectionable error is introduced and built-up until a subsequent signpost can be successfully read.
In the present invention, the scanning system can read signposts that are positioned on both the right and left-hand sides of the street and the redundancy thus built into the system, along with the use of an odometer to measure the distance traveled from the last signpost, minimizes the impact of the problem. Further, in northern areas which receive snow, the situation may occur wherein a driving snow at a certain angle will completely cover the coded signposts on one and only one side of the street to render them unreadable. In such situations, the present apparatus will still provide adequate readings for system operation. The scanning and reading of signposts on both sides of the vehicle's path also insures for a more accurate system in the presence of damaged or missing signposts.